In the related art, it has been widely known that a signal for a passive function is transmitted or received between a vehicle communication device that is mounted on a vehicle, and a portable device in a user's possession. Here, the passive function is a function or the like for locking and unlocking a door or the like (opening and closing member) when a user operates a door handle, a door knob switch, or the like without operating the portable device. In this function, when the user performs a predetermined operation on the door handle or the like, a response request signal, which reaches to a predetermined area is transmitted from a vehicle communication device, and a portable device receiving the response request signal responds with a response signal including authentication information. When authentication is successful, locking or unlocking of the door or the like is performed.
Meanwhile, there is a case where theft or intrusion is performed on a vehicle having the passive function using a technique called relay attack. Here, the relay attack is a technique with which a third person with malicious intent performs illegal actions such as unlocking of a vehicle door by allowing communication between the vehicle communication device and the portable device even when a user having a portable device is outside a predetermined area to which a response request signal transmitted from the vehicle transmission device is able to reach, using a repeater.
As a solution to relay attack, for example, in the JP-A-2006-342545, a keyless entry apparatus is disclosed which can easily determine whether or not there is reception of a request signal due to relay attack. In the keyless entry apparatus, a vehicle side transmission unit includes a plurality of transmission antennas which are disposed at different positions on a vehicle, and a portable device reception unit includes a plurality of reception antennas which are disposed toward different axis line directions. A portable device detects intensity of each of signals in the plurality of reception antennas which are transmitted from the plurality of transmission antennas, and compares intensities of the signals in the reception antennas to each other. Accordingly, when the intensities are approximately equal to each other, it is determined that there is a relay attack by using a fact that intensities of signals received by the portable device through a repeater are equal to each other even if the signals are transmitted from the plurality of transmission antennas. In this case, since the portable device does not transmit an answer signal, a door is not unlocked.
In addition, in JP-A-2010-121297, a smart keyless entry system with improved security for preventing relay attack is disclosed. The smart keyless entry system includes masquerade determination means for determining whether or not a person who is in the vicinity of a vehicle is an authorized user. When it is determined by the masquerade determination means that a repeater of a person who is in the vicinity of the vehicle is not the portable device or an authorized user having the portable device is not in the vicinity of the vehicle, unlocking is inhibited with the masquerade determination means lowering an output value of a response signal with which the portable device responds so that an arrival distance of the response signal becomes substantially equal to an arrival distance of a response request signal that is transmitted from the vehicle.
However, when adopting the countermeasure disclosed in JP-A-2006-342545, if a positional relationship between a portable device and a plurality of antennas is a positional relationship in which compared intensities of a plurality of signals are approximately equal to each other, there is an erroneous determination that relay attack is performed, and thus the portable device does not transmit an answer signal and a passive function does not function. For example, in a case where there are transmission antennas on an inner portion on the front side of a vehicle and an outer portion on the rear side of the vehicle, and a portable device is in the vicinity of a rear door of the vehicle, since signal intensities of the transmission antennas on the inner portion on the front side of the vehicle become approximately equal to those of the transmission antennas on the outer portion on the rear side of the vehicle, it is erroneously determined as relay attack, whereby the passive function does not function, and a door cannot be locked, unlocked, or the like.
In addition, when adopting the countermeasure disclosed in JP-A-2010-121297, when it is determined that an authorized user is not in the vicinity of a vehicle, unlocking is inhibited by a lowering of an output value of a response signal with which the portable device responds so that an arrival distance of the response signal becomes substantially equal to an arrival distance of a response request signal that is output from the vehicle. However, since it cannot be distinguished whether the signals are received directly from the vehicle or through a repeater, it is difficult to determine whether or not the relay attack is performed in the first place.